<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button id="but">发送Ajax请求</button>
    <script>
        var but = document.getElementById('but');
        but.onclick = function(){
            var xhr = new XMLHttpRequest();
            xhr.open('get', 'http://localhost:3000/error');
            xhr.send();
            xhr.onload = function(){
                console.log(xhr.responseText);
                console.log(xhr.status);
                if(xhr.status == 400){
                    alert('请求出错')
                }
            }
            xhr.onerror = function(){
                alert('网络中断，无法发送Ajax请求');
            }
        }
    </script>
</body>
</html>